Build R2RDump from source

Context: R2RDump's code is in dotnet/runtime for 5.0. It brings in a prebuilt dependency from a repo that isn't in source-build.

Currently R2RDump is not included in the Runtime and SDK so we plan to skip building it from source: https://github.com/dotnet/runtime/issues/43599. The project's dependencies don't look trivial to source-build. (Native code, doesn't use Arcade and dependency flow as of writing, code maintenance in flux between dotnet/llilc and dotnet/jitutils. More details in issue.)

But should R2RDump be something that we *do* provide as part of the source-built .NET ecosystem?

I'm not too familiar with it, but it [sounds like](https://github.com/dotnet/runtime/tree/dcc49112aa4b03a2347409fb95e837767e2ad49f/src/coreclr/src/tools/r2rdump) it might be interesting:

> Parses and outputs the contents of a ReadyToRun image

This issue tracks inclusion of R2RDump in source-build. If it's something we will never care about, then we can close, but I think it's worth tracking as it's something that exists in dotnet/runtime but we won't be source-building for now.